The staging area is a lot more useful than what you can do with an interactive commit, since it persists between operations. You can use it to accumulate changes that are "done" while you continue to experiment with some other changes on top, even if you're planning on committing the whole thing at once, and if you wind up not liking your new changes, revert to what you had in the staging area; or you can realize what you're working on will be easier to finish as two commits, stage part of it, stash the other part, finish the part that was staged and commit it, then apply the stashed part and finish that too.
This is a lot more than you get with just a single interactive commit operation.
You could use a commit for this purpose, and continually amend or revert this commit, but in terms of UI, it's helpful to have a set of operations specifically for moving things in and out of the staging area that's different than what you do for manipulating commits on general.